When I think Messenger I think Abbesses... If that is too big though I would definitely entertain the thought of a Musette. It's an awesome bag. Good luck with your decision.
Tulum GM was way too big on me...... so i sent it back, then i bought Saumur in mono instead, one of my fav bag ever! very comfortable and usful! i bought Hudson GM in the past, very good looking bag, but not so comfortable in the shoulder.
i have Musette Tango in Damier --- this was one of my fav BEFORE i had a baby, so comfortable and sharp looking, i used to wear it even when i was driving!